I. Field of the Invention
The present invention relates to wireless communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for scheduling acquisition attempts of one or more service providing systems in a subscriber station capable of operation with a plurality of communication systems.
II. Related Art
As mobile communication systems become more prevalent in society, the demands for greater and more sophisticated service have grown. To meet the capacity needs of mobile communication systems, techniques of multiple access to a limited communication resource have been developed. The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems.
The use of CDMA techniques in a multiple access communication system is known in the art and is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, assigned to the assignee of the present invention. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, also assigned to the assignee of the present invention.
In order for service to be provided using a subscriber station, the subscriber station must first acquire a service providing system. Such service providing systems can be identified, for example, by mode (for example, CDMA, TDMA, GSM, etc.) and frequency. Many wireless subscriber stations (also known as wireless communications devices, user terminals, or mobile stations) include tables and/or lists that are used by a subscriber station when it is attempting to obtain service. For example, a subscriber station can maintain a preferred roaming list (PRL) that is maintained in such a manner that the subscriber station can readily determine which systems (preferred or negative) cover common geographical regions (that is, areas of common radio coverage). Moreover, the PRL can rank systems from most desirable to least desirable. Accordingly, subscriber stations can prioritize their attempts to acquire service such that they attempt to acquire service on more desirable systems before attempting to acquire service on less desirable systems.
Subscriber stations can also include tables or lists of the N (for example, N=10) most recently used systems (that is, the systems that were most recently used to provide service). Such a table or list is typically referred to as a most recently used (MRU) table. A subscriber station can, for example, attempt initial system acquisition (for example, after a power up) using the MRU table.
Subscriber stations typically use one or more of the above discussed tables (and/or other similar types of tables) to determine the order in which to attempt to acquire systems. For example, after power up, a subscriber station may attempt to acquire a system based on an order specified by an MRU table and/or a PRL. Similarly, after a call is dropped (for example, due to loss of signal or signal fading), a subscriber station may attempt to acquire (including re-acquire) a system based on an order specified by the MRU table and/or the PRL. More specifically, upon detecting specific events, such as power up or system loss, a subscriber station may be programmed such that the first system the subscriber station attempts to acquire (specifically in this case, re-acquire) is the last system upon which service was successfully provided (that is, the most recently used system). An example of another event that can be detected is the loss of a paging channel (for example, due to loss of signal or signal fading) while the subscriber station is in idle mode.
For example, after a subscriber station is powered up, the subscriber station first attempts to acquire service using the mode and frequency of the system upon which service was last provided when the subscriber station was last powered on (that is, prior to power down). In other words, the subscriber station first attempts to re-acquire the most recently used system before attempting to acquire one or more other systems. In this example, the acquisition of the most recently used system is attempted because it is likely that the subscriber station has remained within the same geographic region, and thus, that service can again be provided using the most recently used system.
Similarly, after a subscriber station drops a call due to loss of signal or signal fading, the subscriber station immediately attempts to re-acquire service using the mode and frequency of the system upon which service was being provided when the call was dropped. In this example, the re-acquisition of the most recently used system is attempted because it is likely that the cause of the system loss is only temporary, and thus, that service can again be provided using the most recently used system. For example, a subscriber station may lose system acquisition because the user of the subscriber station travels on an elevator for a few seconds, travels behind a service blocking building for a few seconds, or travels in an underground tunnel for a few minutes. In each of these examples, successful re-acquisition of the most recently used system is likely once the user, for example, exits the elevator, is no longer behind the building, or exits the underground tunnel.
Some prior subscriber stations, designed by the assignee of the present invention, are adapted to alternate between the most recently used system and other systems when attempting to acquire (including re-acquire) a system. More specifically, the subscriber station""s first attempt is to acquire service on the most recently used system, the second attempt is to acquire an alternative system, the third attempt to acquire the most recently used system, the fourth attempt is to acquire another alternative system, the fifth attempt to acquire the most recently used system, the sixth attempt is to acquire a further alternative system, and so on. The order in which the subscriber station attempts to acquire service on the alternative systems can be based on various algorithms that are known in the art. Once all alternative systems are attempted in accordance with one of such algorithms, the above described alternating with the most recently used system is stopped and acquisition is attempted based only on an algorithm that does not give such repeated high priority to the most recently used system. Even though the above described method of alternating between the most recently used system and alternative systems (referred to hereafter as the xe2x80x9calternating methodxe2x80x9d) has proved useful, this alternating method has some drawbacks.
First, since the alternating method is based on an alternating sequence scheme, it is dependent on the processing speed of the subscriber station""s hardware. More specifically, if a subscriber station operates at a high speed, the subscriber station may alternate between the most recently used system and all of the hypothesized available alternative systems in only a few seconds. As mentioned above, the alternating method stops after unsuccessful attempts to acquire all the hypothesized available alternative systems. Thus, the subscriber station may stop giving the most recently used system repeated high priority earlier than it should. For example, while the subscriber station is located in an elevator, the subscriber station may complete the alternating method and stop giving the most recently used system repeated high priority even though the most recently used system would be immediately available once the subscriber station exits the elevator. In summary, the subscriber station forgets about the lost system too quickly.
Second, since the alternating method is based on an alternating sequence scheme, it may give too much priority to the most recently used system, thereby wasting acquisition attempts and slowing down the amount of time necessary to acquire an alternative system. For example, assume a user of a subscriber station walks behind a large building that blocks service (or enters an elevator or tunnel) thereby causing the subscriber station to drop a call. Using the alternating method, the subscriber station may, for example, attempt to acquire the most recently used system three separate times, and alternative systems three times, within less than a second (depending on the processing speed of the subscriber station). However, it is unlikely that the subscriber station will be able to acquire the most recently used system if, for example, 0.3 seconds earlier the subscriber station was unable to acquire that system. In summary, the subscriber station spends too much time attempting to re-acquire the lost system, while starving other systems.
Accordingly, there is a need to improve the way subscriber stations attempt to acquire (including, re-acquire) systems.
The present invention is directed toward methods, apparatuses, and computer program products for improving the way a subscriber station attempts to acquire a service providing system. The present invention gives appropriate priority to specific systems (such as the most recently used system) without giving undue or too much priority to such specific systems.
A method of the present invention includes the step of detecting an event that triggers use of a system acquisition schedule. The system acquisition schedule defines when to attempt to acquire a desired system. The subscriber station attempts to acquire the desired system in accordance with the system acquisition schedule. The subscriber station performs other functions when the system acquisition schedule does not indicate to attempt to acquire the desired system. The present invention can also include the step of selecting the system acquisition schedule from a plurality of system acquisition schedules, wherein the selecting is based on the triggering event.
According to the present invention, a subscriber station can attempt to acquire more than one desired system in accordance with more than one system acquisition schedule. For example, while attempting to acquire a first desired system in accordance with a first system acquisition schedule, the subscriber station can detect another event that triggers use of a second system acquisition schedule. The second system acquisition schedule defines when an attempt should be made to acquire a second desired system. The subscriber station can then also attempt to acquire the second desired system in accordance with the second system acquisition schedule.
The subscriber station may attempt to acquire other systems when the system acquisition schedule does not indicate an attempt should be made to acquire the desired system. When attempting to acquire other systems the subscriber station follows alternative algorithms for prioritizing systems. It is possible that one of the xe2x80x9cotherxe2x80x9d systems can be the desired system (if, for example, a PRL specifies an attempt should be made to acquire that system). However, in a specific embodiment, each of the other systems is different from the desired system. For example, if each system is defined by a mode/frequency combination, and the desired system is defined by a specified mode/frequency combination, then each of the other systems is defined by different mode/frequency combinations than the desired system.
System loss can be the event that triggers use of the system acquisition schedule. In this situation, the system acquisition schedule can define when an attempt should be made to reacquire the desired system whose loss triggers use of the system acquisition schedule. A power up of a subscriber station is an example of another triggering event. In this situation, the system acquisition schedule can define when an attempt should be made to acquire the desired system, the desired system being the system that was most recently used by the subscriber station.
A system acquisition schedule can be time or sequence based. More specifically, a system acquisition schedule can define specified times at which the subscriber station should attempt to acquire the desired system. Alternatively, a system acquisition schedule can define specific sequence counts according to which the subscriber station should attempt to acquire the desired system. The subscriber station performs other function at times or counts other than the specified times or counts defined by the system acquisition schedule. The xe2x80x9cother functionsxe2x80x9d can be, as mentioned above, attempting to acquire other systems.